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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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HIDDEN, FOURTH AND SMALLER GLACIERS                  155
the marginal stream had cut a distinct rock gorge (PI. LXXXIV), from 2 to 8 feet in depth, contouring the hillside. It cannot be conceived that on this steep mountain slope a stream would follow the contours for several hundred feet, unless some barrier interfered with its flow down the hillside. Evidently, therefore, this rock gorge was the result of marginal drainage. We cannot be certain that the channel may not have started at some earlier stage in a high level of Hidden Glacier, though the coincidence of position at the exact margin of the present stand of the glacier would in that case be quite remarkable. The shallowness of the gorge, and indications of its newness, have led us to believe that it is the product of marginal drainage at the present stand of Hidden Glacier.
On the south side of the Hidden Glacier valley the mountain walls rise more abruptly, and there is an absence of gravel slopes such as exist on the northern side. Only for that reason the marginal conditions on the south side are quite different from those on the north. There is also the factor of difference in power of sunshine on the two sides of the valley. On the moderately-sloping north side the low-lying sun strikes with power,' but on the steeply-rising south side the sun's rays produce little direct effect. Indeed, at the mountain base for a considerable part of the day in summer this side of the valley is in shadow. As a result of these different conditions on the two sides, there was as yet no marginal valley there, and the ice extended with uniform slope right up to the mountain base. In fact, as late as the middle of July, there was a slope upward from the ice surface caused by the snow banks which had slid from the mountain side down to the glacier, which had not yet melted. By comparison of the photographs of 1909 with those of the previous years it became evident that the amount of snow remaining on the mountain slopes just above the glacier was much greater in the latter year than in the earlier years. The advance of the glacier and the rising of its surface introduced a temporary change in the local climate. Both on the south and the north margins there had as yet been too little ablation to have brought out in relief the lateral moraines which will doubtless ultimately develop again on both sides of the Hidden Glacier.
The very front of the Hidden Glacier sloped moderately, almost to the edge, with some low, debris-covered ice cones just back of it on the glacier surface, then ended in a low ice cliff from 20 to 80 feet in height. The ice front projected farthest in the southern half of the valley. From the ice front issued many small streams and two good-sized ones, the larger, as in 1905, issuing from the south side very close to the valley walL The next largest stream came out 100 yards from the northern margin, and was evidently carrying not only drainage from the ice, but also the marginal drainage of the north side of the glacier which in its lower course flowed through a tunnel in the ice. Fresh faulting by slumping of the ice surface near where this north stream emerged, showed clearly that its drainage was in process of development, and the extension of this slumped area towardd the northern margin of the glacier is proof of the direction from which the supply for this stream was derived. Both the south and north streams flowed out upon the gravel plain in front of the glacier, and thence down to the sea; but between these two streams there was a much smaller stream with water derived from a number of small streams issuing from the steep ice front, at first flowing in a shallow valley parallel to the ice front, the streams from the two ends of the valley finally uniting in about the center of the glacier and going to the sea as one torrent.